The present invention relates to ionic conductors, processes of producing same and devices that include same. More specifically, the present invention relates to an ionic conductor that includes a polymer containing carbon clusters, processes for producing same, and electrochemical devices provided with the ionic conductors.
There is an increasing demand for a high-functional ion-conducting membrane. Particularly, a high-functional protonic conductor is in strong demand in the field of fuel cell.
A fuel cell with a solid electrolyte of proton-conducting membrane is constructed such that the proton-conducting membrane is held between the fuel electrode and the oxygen electrode and electromotive force resulting from reaction between fuel and oxygen is evolved across the fuel electrode and oxygen electrode.
In a fuel cell that consumes hydrogen gas as fuel, hydrogen gas fed to the fuel electrode is oxidized by the reaction shown below to give electrons to the fuel electrode:2H2→4H++4e−
The resultant hydrogen ions H+ (protons) move or migrate to the oxygen electrode through the proton conducting membrane.
The hydrogen ions that have moved to the oxygen electrode react with oxygen fed to the oxygen electrode, thereby giving rise to water and take electrons from the oxygen electrode, by the reaction shown below:O2+4H++4e−→2H2O
Fuel cells are attracting attention as a new environment-friendly electric power generator and are being developed in various fields because of their efficient energy conversion (from fuel's chemical energy to electrical energy) and their freedom from emitting environmental pollutants, such as nitrogen oxides.
The fuel cells mentioned above are roughly divided by the kind of proton conductor used therein because their performance (operating temperature and conditions) depends largely on the properties of the proton conductor. Therefore, improvement in the performance of proton conductor is essential to improvement in the performance of fuel cells.
Fuel cells designed to operate at temperatures above normal temperature and lower than 100° C. usually employ a proton-conducting polymeric membrane formed from a solid polymer. Typical examples of such a membrane are Nafion® from DUPONT and GOA membrane from GOA INC., which are made of perfluorosulfonic acid resin. They are still under development. In addition to these known membranes, new proton-conducting polymeric membranes derived from hydrocarbons have recently been reported in academic societies and journals.
Proton conductors which have become known recently are polymolybdic acid (H3M12PO40.29H2O, M=Mo or W) and metal oxides (such as Sb2O5.nH2O, n=5.4 in general), both containing water of hydration in large amount. These types of polymeric materials and hydrated compounds exhibit high proton conductivity at or near normal temperatures when they are placed in a moist environment.
In the case of water-containing polymeric material (such as perfluorosulfonic acid resin), proton conductivity manifests itself as the result of protons released from sulfonic acid groups moving through water abundantly present in the polymer matrix. Consequently, in order to maintain high proton conductivity of the perfluorosulfonic acid resin, it is necessary to continuously supply water during operation, thereby keeping the resin sufficiently wet. A proton conductor of inorganic metal oxide also decreases rapidly in proton conductivity in its less moist state.
For this reason, fuel cells that employ a proton conductor as a proton conducting membrane require a humidifier to supply water to gas being fed to fuel cells and auxiliary equipment to control the water content. This makes the system complex and large, which leads to a higher installation cost and operating cost.
Moreover, the disadvantage of the fuel cells that employ a water-containing polymeric material (such as Nafion®) is that the polymeric material keeps water therein such that the water phase is separate from the hydrophobic polymer skeletons. Water in such a state is subject to evaporation at high temperatures and freezing at low temperatures. Consequently, such fuel cells are limited in the range of their operating temperatures, because they need provisions to prevent water from boiling and freezing. In addition, the hydrated state is unstable and dependent largely on temperature, and hence the proton conductivity is greatly affected by temperature and other environmental conditions.
International Patent Publication No. WO 01/06519 A1 discloses a new material that exhibits proton conductivity in a dry state. This new material is composed mainly of carbon clusters (which have a unique molecular structure, such as C60 and C70 fullerenes and nanotubes) doped with proton-dissociating groups.
As disclosed, the term “proton-dissociating group” refers to a functional group which permits a hydrogen atom to electrolytically dissociate in the form of proton (H+) and release itself from the group. The term “functional group” refers to not only atomic groups having only one bonding hand [site] but also atomic groups having more than one bonding hand [site]. The “functional group” may be connected to the end of the molecule or present in the molecule. These definitions shall apply also to the present invention.
The above-mentioned fullerene derivative increases in proton conductivity with the increasing number of proton-dissociating groups introduced into each fullerene molecule. However, as the number of proton-dissociating groups introduced into fullerene molecules increases, the fullerene derivative becomes more water-soluble because the proton-dissociating groups are hydrophilic. When used as an electrolyte of fuel cells, such a water-soluble fullerene derivative would dissolve in water evolved by electrode reactions during power generation.
Thus, there is a tradeoff between imparting high proton conductivity to fullerene derivatives and maintaining the fullerene derivatives to be less soluble in water. Fullerene derivatives as an electrolyte of fuel cells need careful design and material selection regardless of whether or not they are used alone or in combination with other materials.
In addition, fullerene-based proton conductors are mostly in powder form. This can create problems with respect to the production of same into films.
A need therefore exists to provide improved ionic conductors.